Nonaqueous electrolyte-type electrical double-layer capacitors have the characteristic of being chargeable and dischargeable at a large current, and thus are promising as energy storage devices for electric cars, auxiliary power supplies and the like.
Nonaqueous electrolyte-type electrical double-layer capacitors are composed of, for example, a positive/negative pair of polarizable electrodes and a nonaqueous electrolyte. The nonaqueous electrolyte is generally composed of an electrolyte salt and a nonaqueous organic solvent. Many studies have hitherto been conducted on combinations of these electrolyte salts and nonaqueous organic solvents. Quaternary ammonium salts (see, for example, Patent Documents 1 to 3), quaternary phosphonium salts (see, for example, Patent Document 4) and the like are widely used as electrolyte salts because of their solubility and degree of dissociation in organic solvents and because of their broad range of electrochemical stability.
However, because these electrolyte salts contain halogen atoms such as fluorine atoms on the anions, they pose problems in terms of the environmental impact. Improvements are desired in this respect.
Electrolyte salts used in electrochemical applications such as electrolytes for nonaqueous electrolyte-type electrical double-layer capacitors are required to have electrochemical properties such as a high ionic conductivity and a wide potential window. However, electrolyte salts which are halogen-free and have a wide potential window have not hitherto been known.